British light-heavyweight Joshua Buatsi earned a win yet underwhelming debut to his career under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a competitive 10-round bout at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful individual punches in a match where the styles failed to mesh, with excessive holding spoiling the action.
The officials judged it 96-94 to the winner, with the third a 95-95.
"It wasn't my greatest display but what defines champions? They win," the victor said in the ring as he was faced boos from the spectators.
The Londoner got back to winning after his maiden career setback - an thrilling contest with his previous opponent in last winter - to earn a twentieth professional win.
Various analysts believed Parker, 31, deserved the win but the British boxer experienced the second loss of his 28-match professional journey.
The Champion Earns Triumph in Uneventful Match
Buatsi's progress has plateaued in recent years through promotional disputes and extended periods of lack of fights.
Earlier forecast as the future of the British fight scene, he now finds himself trying to achieve expectations - perhaps in what appears to be a last chance - under fresh management.
He started carefully, with both competitors ready to take risks in a tentative initial rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after damaging his hand mid-fight, was participating in his second bout at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for the favorite, who worked behind a tight guard to barely take the more effective rounds.
The crowd's patience diminished amid regular grabbing, with the challenger falling to the floor multiple occasions during clinches.
The underdog closed well, but several stanzas were hard to judge. However, former world champion the boxing analyst - commentating on the broadcast network - had it particularly wide for Parker, giving Buatsi just one round.
Buatsi moves on, but with the presence of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Tribute Motivates All-Manchester Battle
In the supporting bout, Lyndon Arthur overcame Bradley Rea by divided decision in a outstanding all-Manchester fight to win the continental belt.
The stadium grew hushed for a deeply poignant memorial before the fight as ten bells rang in honor of Manchester's own Ricky Hatton, who succumbed at age 46 in last month.
It was the initial significant broadcast boxing event in the urban center since the passing of Hatton. As the final chime disappeared, the stillness gave way to a chant of "there's just one Hatton".
The fighting within the ring equaled the emotion. Arthur was knocked down by a powerful punch squarely on the jaw in the early round, standing uncertainly to his feet on the referee's count.
But the traditional red shorts of Arthur soon advanced again. From the other corner, Rea - in rival City blue - was damaged quickly, noticeable damage developing under his eye as the pair traded punishing blows.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with stinging uppercuts as the intensity rarely dropped over 12 rounds.
Arthur edged the result with official scores of even score, 115-113 and wider gap, but both fighters were relentless and courageous, all the qualities their idol Hatton represented.
